Niger's President Mahamadou Issoufou was in Paris on Monday for talks with his French counterpart François Hollande. He will also have talks in London with David Cameron about the Sahel hunger crisis. Photograph: Chamussy/Sipa/Rex

Relief groups are stepping up their appeals for aid to tackle the worsening food crisis in west Africa, where more than 18 million people face hunger.

Save the Children, which has increased its emergency operations in the Sahel, on Tuesday said it faces a funding shortfall of almost £26m. The charity hopes to close the funding gap and raise extra money to help the 1.5 million people – including almost a million children – most urgently in need.

"For months now, families have been telling us they have next to nothing to eat," said Justin Forsyth, chief executive of Save the Children. "In Niger, mothers have little or no food to feed their children. Our analysis now shows just how bad the situation has become and confirms our worst fears: a major emergency is now upon us."

On Monday, Unicef, the UN agency for children, doubled its appeal for funds to £153m to help children affected by the food crisis in the Sahel. Unicef said it aims to address the most pressing needs, including preventing epidemics through vaccination campaigns, and reducing the risk of malaria through distribution of bed nets. So far the agency has secured £59m for its emergency response.

The UN says about 18 million people are affected by a drought and food crises in nine countries. Unicef warned in December last year that more than 1 million children would need life-saving treatment for severe acute malnutrition and appealed for $119.5m. The figure has since gone up, as conflict in Mali has forced 170,000 people from their homes, with some seeking refuge in neighbouring Mauritania, Burkina Faso and Niger.

Save the Children, which worked with government, UN and other aid partners, said its analysis shows that in parts of Mauritania the poorest households are missing an estimated 80% of their basic needs for the months of June and July – and have already faced a deficit since January.

In one area of central Mali, the poorest families have less than 40% of the food they need to survive from June to September. As of this month in Burkina Faso, the poorest families will only have 30% of what they need for the next four months – unless they get help immediately. Save the Children experts warned that unless families get that help they will be forced to take drastic measures to survive, selling their remaining assets to buy whatever food they can until they have nothing left to sell.

"The time to act is now," said Forsyth. "We are asking donors not to wait any longer. Any further delays are sure to cost additional lives of children whose deaths we know how to prevent – and can prevent, if we have the means."

Mahamadou Issoufou, the president of Niger, will have talks in London with the UK prime minister, David Cameron and aid agencies this week about the worsening situation. Issoufou, who was in Paris on Monday for talks with the French president, François Hollande, said west African countries will seek a UN security council mandate for military intervention in Mali to deal with Islamist militants.

Issoufou said Afghan and Pakistani Islamist groups were training recruits in northern Mali, which he described not just as a local, but an international threat.

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